The invention relates to a controller for an automatic transmission for use in automobile vehicles, in particular, to a control system which comprises an anomaly detecting means for detecting anomalies occurring on a speed signal line and on signal processing circuits, and which sets the transmission at a predetermined speed stage for the purpose of safety when the anomaly is detected.
An automatic transmission control system having been generally used includes a control circuit which receives a speed responsive signal and a signal indicative of output torque or throttle opening of the engine as input signals, compares both signals with each other, and then outputs a speed range control signal to the transmission. The speed responsive signal is, for example, supplied from a well known vehicle speed sensor of the reed switch type which generates pulses in response to rotation of a drive shaft. A reed switch incorporated in the speed sensor is opened or closed in accordance with the magnetic field generated by a magnet rotating together with the drive shaft. The vehicle velocity sensor is normally disposed at a position near the drive shaft or within a speed meter, but there may occur disconnections in a signal line between the reed switch and the control circuit. In the event of such disconnection, the signal line generally assumes the same electric potential as that which occurs while the vehicle is stopped. When using the reed switch with one end thereof connected to a reference potential or a ground potential and the other end connected to a positive constant voltage source, for example, the signal line will remain at ground level when the line is short-circuited to ground or the reed switch sticks at the closed state, for example, due to the formation of deposits thereon, whereas the signal line will remain at a positive voltage when the line is disconnected or the reed switch sticks at the opened state due to breakage thereof. Thus, the speed responsive signal assumes the same level as that which occurs while the vehicle is stopped. At such occasion, the control circuit in the automatic transmission control system makes a decision that the vehicle has stopped, and hence issues the speed range control signal allowing the transmission to be shifted to the speed step for a low speed (for example, the 1st speed). However, should the vehicle be traveling at a high speed, the abrupt shift down to the 1st speed entails a sudden engine braking, thereby resulting in a large shock which may damage the transmission or engine.
Such a problem can be solved by an improved system which includes an anomaly detecting means adapted to detect anomalies occurring in a signal line from a decision reference indicator detecting means such as a vehicle speed detecting switch or a throttle opening detecting switch to the control circuit making a decision on speed range, and which can set and lock the automatic transmission at a safety speed range in the anomalous condition. However, depending on the conditions while traveling, it sometimes occurs that a signal similar to that obtained in the anomalous condition will appear even if the signal line operates normally. For example, when the vehicle traveling at a high speed is subjected to emergency braking and the wheels of the vehicle stop their revolutions, the vehicle speed signal assumes a low level similarly to the case where the signal line is disconnected or short-circuited. On such an occasion, it is preferable to promptly release the anomalous setting (safety locking) which otherwise would be maintained in the anomaly detecting means and the control circuit.